1. Field of the Invention
The present invention relates to a clamping device and a method for clamping an object on a support. The present invention further relates to a lithographic apparatus and a method for loading a substrate on a substrate support of a lithographic apparatus. Finally, the present invention relates to a machine readable medium.
2. Description of the Related Art
A lithographic apparatus is a machine that applies a desired pattern onto a substrate, usually onto a target portion of the substrate. A lithographic apparatus can be used, for example, in the manufacture of integrated circuits (ICs). In such a case, a patterning device, which is alternatively referred to as a mask or a reticle, may be used to generate a circuit pattern to be formed on an individual layer of the IC. This pattern can be transferred onto a target portion (e.g., including part of one or several dies) on a substrate (e.g., a silicon wafer). Transfer of the pattern is typically via imaging onto a layer of radiation-sensitive material (resist) provided on the substrate. In general, a single substrate will contain a network of adjacent target portions that are successively patterned. Conventional lithographic apparatus include so-called steppers, in which each target portion is irradiated by exposing an entire pattern onto the target portion at once, and so-called scanners, in which each target portion is irradiated by scanning the pattern through a radiation beam in a given direction (the “scanning”-direction) while synchronously scanning the substrate parallel or anti-parallel to this direction. It is also possible to transfer the pattern from the patterning device to the substrate by imprinting the pattern onto the substrate.
In the known lithographic apparatus each substrate to be exposed, is loaded on a substrate support on which the substrate is supported during the exposure of a patterned beam of radiation. To clamp the substrate on the substrate support a clamping device is provided. In a known embodiment of the lithographic apparatus a vacuum clamping device is used. Such a vacuum clamping device provides a vacuum force with which the substrate is clamped on the supporting surface of the substrate support. In the case a substrate is straight, the substrate will be clamped on the support surface without any substantial internal stresses in the substrate.
However, substrates may not be straight, but for instance be warped in a number of shapes, such as a corrugated shape, cylindrical shape, dome shaped, a saddle form or another shape. This may be caused by the production method used to make the substrate, or by pre- or post exposure processes to which the substrates are subjected during the manufacture.
When a warped substrate, for instance a dome-shaped substrate is clamped on a substrate support for instance by means of a vacuum clamp, the substrate may first contact with the substrate support at the outer circumference of the substrate and thereafter over the rest of the surface of the substrate. Due to the clamping force the substrate is forced into a substantially straight form, while the clamping is started at the outer circumference of the substrate. As a result, stresses may be induced in the substrate when it is clamped on the supporting surface. In this application ‘warped’ object will refer to any such shapes as cylinder, saddle, or other unwanted deformation of the shape of the object.
These stresses may have a negative influence on the final product quality. Also, since the substrate is clamped in another form than desired, the overlay performance of the projections of the lithographic apparatus may decrease which may have a negative influence on product quality.